Temperature stabilized gyromagnetic element



3,246,263 TEMPERATURE STABILIZED GYROMAGNETIC ELEMENT Filed March 29,1963 J. G. CLARK April 12, 1966 2 Sheets-Sheet l 100 AXIS HARD 111 AXISEASY 110 PLANE FIG.2.

INVENTOR /0/-//V G CLARK QAmkM ATTORNEY April 12, 1966 J, CLARK3,246,263

TEMPERATURE STABILIZED GYROMAGNETIC ELEMENT Filed March 29, 1965 2Sheets-Sheet 2 ROOM TEMP 0 3O RE'hATIVE ELD STRENGHT INVENTOR. J0H/v GACLARK ATTORNEY United States Patent Delaware Filed Mar. 29, 1963, Ser.No. 269,034 4 Claims. (Cl. 333-241) This invention relates totemperature stabilization of electromagnetic wave devices usingmagnetically polarizable gyromagnetic materials, and more particularlyrelates to the substantial elimination of variations in the magnitude ofthe effective internal magnetic field within the polarized material dueto the change or drift in magnitude, as a function of temperaturechange, of the anisotropy field in an elliptically shaped member of agyromagnetic material having a single cubic crystal structure.

Magnetically polarizahle gyromagnetic materials are used quiteextensively in electromagnetic wave devices and circuits because oftheir ability to introduce into the circuits reciprocal ornon-reciprocal coupling, attenuation, phase shilit, power limiting,frequency conversion, etc. The effective internal magnetizing field Hwithin a magnetically polarizable gyromagneti-c material ordinarily is afunction of the saturation magnetization M of the material, the appliedbiasing magnetic field H the anisotropy field H associated with theparticular material, and the demagnetization fields H which are afunction of the shape of the specimen of the material used. Thesaturation magnetization M the anisotropy field H and thedemagnetization fields H all are temperature dependent so that theinternal magnetization field varies as the temperature of the materialchanges due to changes in ambient temperature, and/or because of therise in temperature resulting from the absorption of electromagneticwave energy within the material. This change in internal magnetizationfield is undesirable because it results in changes in the operatingcharacteristics of the material and thus of the device or circuitemploying the material.

In recent years small, highly polished, single crystal spheres ofgyromagnetic materials have been used in devices such as gyromagneticcouplers to perform the functions of power limiters and filters, forexample. The small spheres have been used at least partially because theeffects of the saturation magnetization term M and the demagnetizationfields H, are substantially eliminated because of the spherical shape,thus eliminating the problem associated with temperature dependence ofthese two quantities. The temperature dependent variations in theoperating characteristics of small, highly polished spheres of singlecrystal material are not entirely eliminated in known devices, however,because the anisotropy field associated with the crystalline structureof the material still is temperature dependent, and in devices employingthe single crystal spheres as coupling elements between two waveguides,for example, the problem still is of concern. The reason for this is asfollows. The single crystal sphere of gyromagnetic material ismagnetized to its gyromagnetic resonance condition in order toefficiently accomplish its coupling action, and since the resonance linewidth of a single crystal sphere of yttrium iron garnet, or similarcubic crystal material that commonly is employed, is very narrow, anyslight change in the internal magnetization field of the materialresults in a change in the resonant frequency of the mate rial, thussignificantly affecting the gyromagnetic coupling action performed bythe spheroid of material.

It therefore is an object of this invention to temperature stabilize theoperation of an electromagnetic wave device employing a small, highlypolished sphere of a gyromagnetic material having a single cubic crystalstructure.

Another object of this invention is to substantially eliminatevariations in operating characteristics, due to temperature changes, inan electromagnetic wave device employing a gyromagnetic material havinga single cubic crystal structure by eliminating from the internalmagnetic field within the material the effects caused by the variationsin the anisotropy field resulting from the temperature changes.

A further object of this invention is to eliminate the influence oftemperature dependent anisotropy field changes on the internal magneticfield of a magnetically polarized single cubic crystal ellipsoid ofgyromagnetic material.

In accordance with this invention, the change in the internalmagnetization field of a small spheroid of a single cubic crystalgyromagnetic material that results from the change or drift in magnitudeof the anisotropy field associated with the crystal is substantiallyeliminated by aligning the crystalline structure of the material withrespect to the biasing magnetic field so that said biasing field isapplied parallel to the 110 plane of the crystal and is inclined at anangle of approximately 30 to the 100 axis of the crystal.

The present invention will be described by referring to the accompanyingdrawings wherein FIG. 1 is a simplified illustration of anelectromagnetic wave device whose performance may be improved inaccordance with the teachings of the present invention;

FIG. 2 is a diagrammatic illustration showing the crystallographicorientation of a single cubic crystal of a gyromagnetic material withrespect to the direction of the applied biasing magnetic field so as toeliminate the temperature variations in the strength of the internalmagnetic field that are caused by the temperature dependent anisotropydrift commonly associated with these materials; and

FIG. 3 is a graphic illustration substantiating the improved resultsachieved in a device constructed in accordance with this invention.

Referring now in detail to the accompanying drawings, thecrystallographic alignment of an ellipsoid of gyromagnetic materialhaving a single cubic crystal structure, as taught by this invention, isparticularly useful in a gyromagnetic coupling limiter device of thetype illustrated in FIG. 1. This device is adapted to propagateelectromagnetic waves in the TEM mode and is comprised of a shortcylindrical body member 10 having coaxial line input and outputconnectors 11 and 12 disposed at with respect to each other on thecylindrical surface of body member 10. Conductive bottom plate 14completely encloses the bottom portion of body mem ber 10, and a similarconductive plate, not shown, is adapted to enclose the other end of bodymember 10. The inner conductors 17 and 18 of coaxial line connectors 11and 12 extend through apertures in body mem ber 10 and connect with thinconductive strips 20 and 21 which extend radially in overlappingrelationship with respect to each other. Disposed between conductivestrips 20 and 21 and in intimate contact therewith, is a small highlypolished ellipsoid or sphere 22 of a low loss single cubic crystalgyromagnetic material such as yttrium iron garnet, gallium substitutedyttrium iron garnet, lithium ferrite, or a similar single cubic crystalmaterial suitable for use in known devices of this type. As is wellknown, the spheroid of gyromagnetic material functions in a mannersimilar to a resonator to couple electromagnetic wave energy betweenconductive strips 20 and 21. In the typical use of the device of FIG. 1as a gyromagnetic coupling resonator, high power pulses ofelectromagnetic waves may be coupled into coaxial line input terminal 11and are coupled from strip conductor 2% through single crystal sphere 22to strip con ductor 21. The spheroid 22 of gyromagnetic materialoperates in response to this high power pulse to couple to outputcoaxial connector 12 only the energy up to some threshold power level,the remainder of theenergy above this threshold level being dissipatedin the spin waves associated with the precessing magnetic moments of theelectrons of the gyromagnetic material. To operate in this manner thesingle crystal sphere 22 ofgyromagnetic material is magneticallypolarized or biased in a direction transverse to the plane of the paperto its g'yromagnetic resonance condition. The single crystal sphere ofyttrium iron. garnet, and other similar materials of single crystalcubic structure, is characterized.

by having a very narrow gyromagnetic resonance line width, and anyetfect, such as a change in the internal magnetization field of thespheroid, which tends to change the gyromagnetic resonance frequency ofthe sphere, has an appreciable effect on the operating characteristicsof the device. One reason for using the spherical shape for the elementof gyromagnetic material is to eliminate the demagnetization fields andthe influence of saturation magnetization M from the gyromagneticresonance phenomenon since both of these quantities are temperaturedependent and would cause a change in the internal magnetization of thesphere, and thus a change in the resonant frequency of the sphere as itstemperature increases due to absorption of the electromagnetic Waveenergy experienced as a result of its power limiting action.

With the saturation magnetization M and the demagnetizing fieldseliminated from the gyromagnetic resonance equation, the effectiveresonance field for the single crystal sphere may be expressed in thissimplified form:

ert mnll o+ annl wherein H is the effective internal magnetic fieldrequlred for gyromagnetic resonance,

H is the external magnetizing field applied parallel to the 110 plane ofthe crystal.

H is the anisotropy field associated'with the cubic crystal structure ofthe material and is equal to K 4TrM where K is the first orderanisotropy constant and M 7 is the saturation magnetization of thematerial,

magnetic field H is substantially eliminated so that the I efiectiveinternal resonance field H is a function only of the applied magneticfield H which is, or can be made, substantially independent oftemperature.

The effect of the anisotropy drift on the effective inwith respect tothe applied magnetic field H so as to minimize this effect. This desiredorientation may be arrived at as follows. Expanding Equation 1 aboveresults in the following expression:

Further, theoretical and experimental evidence indicates that the Htermis small with respect to the other terms in the equation and may bedropped. Equation 2 therefore reduces to Since the second term on theright side of Equation 3 is the only one involving the anisotropy fieldthat has temperature dependent characteristics, the desired temperaturestable operation of a device employing the single crystal sphere may beobtained if this term can be eliminated. This indeed may be accomplishedby setting the quantity (A-l-B) equal to zero. This term reduces to zerowhen the value of in the expressions. for A and B is equal to 29 degreesminutes.

FIG. 2-is a diagrammatic illustration showing the angular alignment ofthe magnetic biasing field H with respect to the crystallographicstructure of a single crystal sphere of yttrium iron garnet or othersuitable gyromagnetic material having a single cubic crystal structure.As illustrated in FIG. 2, the magnetic biasing field H is 7 appliedparallel to the 110 plane of the cubic crystal and ternal resonancefield H is dependent upon the orienta- 7 tion of the crystallinestructure of the gyromagnetic sphere with respect to the direction ofthe applied magnetic field 1-1 and in accordance with this invention,the crystal structure of the single crystal sphere is oriented isinclined at an'angle of 29 degrees 40 minutes to the 100 axis of thecrystal. When the specimen of the single cubic crystal gyromagneticmaterial is crystallographically oriented with respect to the externalbiasing or polarizing field in the manner illustrated in FIG. 2, thetemperature variations in the anisotropy field no longer have anyappreciable effector influence on the magnitude of the effectiveinternal resonance field H and the operation of an electromagnetic wavedevice that utilizes the crystal is substantially stabilized againsttemperature changes. i

The inclination of the magnetic biasing field H at an angle of 29degrees 40 minutes with respect to the 100 axis, which is the hard axisof magnetization, represents a departure from known practice wherein itis customary to apply the magnetizing field H parallel to the 111 axisof the crystal, which is the easy axis of magnetization. The priorpractice of magnetizing the crystal parallel to its 111 axis wasfollowed in an attempt to reduce the required strength of the appliedmagnetizing field. However, as pointed out above, this leaves aneffective H term in the'gyromagnetic resonance equation, and this termwhich is subject to change in magnitude with changes in temperature ofthe material causes a resultant change in the value of effectiveinternal magnetic field H I r The effectiveness of the above-describedmeans for temperature stabilizing'the operation of an electromagneticwave device employing a single crystal sphere of yttrium iron garnet,for example, has been verified in practice and typical results areillustrated in the graph of FIG. 3 which is a polar plot, for varioustemperatures, of the effective resonance field H as a function of theangle between the direction of the magnetizing field H and the 100 axisof a single cubic crystal. As may be seen from FIG. 3, the effectivefield for resonance H varies considerably with temperature. It will benoted, however, that the curves for the three temperatures, roomtemperature, 55 C., and C. intersect whenever the direction of themagnetizing field is at an angle-of approximately 30 with respect to theaxis of the crystal, which on the graph of FIG. 3 is the O180 axis, thusindicating that a change in operating temperature of the gyromagneticmaterial will have substantially no effect upon the operation of thedevice in which the single crystal sphere of material is utilized.

The above discussion assumes, of course, that the material is operatingat temperatures below its Curie temperature.

While spherically shaped specimens of gyromagnetic materials presentlyare preferred for use in devices of the type illustrated in FIG. 1,there are other shapes that result in the elimination of demagnetizationfields. The teachings of this invention may be applied to these othershaped specimens as Well.

In practice, any known means for identifying the respective axes of thecrystal may be employed. In practice, I have used the well known X-rayalignment method with considerable success. Other known methods may beemployed as well, without departing from the practice of this invention,since the method of identifying and aligning the crystal form no part ofmy present invention.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes within the purviewof the appended claims may be made without departing from the true scopeand spirit of the invention in its broader aspects.

What is claimed is:

1. In an electromagnetic wave device employing magnetized gyromagneticmaterial whose anisotropy field is substantially stabilized againstchanges due to temperature changes, the combination comprising.

electromagnetic wave supporting means,

means for coupling electromagnetic waves into said wave supportingmeans,

a specimen of a single cubic crystal of gyromagnetic material thatexhibits gyromagnetic effects to said electromagnetic waves, saidspecimen being subject to temperature variations,

said specimen being positioned within said wave supporting means in thepath of the magnetic field of said waves and being magneticallypolarized in a given direction,

said specimen being crystallographically oriented relative to said givendirection to be magnetically polarized parallel to its 110 plane and atan angle of approximately 30 degrees to its 100 axis thereby totemperature stabilize said anisotropy field, whereby the electricalproperties of said device are stabilized against said temperaturevariations.

2. The combination claimed in claim 1 wherein said specimen ofgyromagnetic material is ellipsoidal in shape.

3. The combination claimed in claim 1 wherein said specimen ofgyromagnetic material is a spherically shaped element.

4. The combination claimed in claim 1 wherein said specimen ofgyromagnetic material is magnetically polarized parallel to its 110plane and at an angle of 29 degrees minutes to its axis.

References Cited by the Examiner Yager et al.: Article, FerromagneticResonance in Nickel Ferrite, Physical Review, vol. 8, No. 4, Nov. 15,1950; pages 744-748 relied on.

Lax et al.: Microwave Ferrite, Lincoln Lab., pub. McGraw-Hill, copyright1962; pages 690-692 relied upon.

HERMAN KARL SAALBACH, Primary Examiner.

ELI LIEBERMAN, Examiner.

W. K. TAYLOR, P. GENSLER, Assistant Examiners.

1. IN AN ELECTROMAGNETIC WAVE DEVICE EMPLOYING MAGNETIZED GYROMAGNETIC MATERIAL WHOSE ANISOTROPY FIELD IS SUBSTANTIALLY STABILIZED AGANIST CHANGES DUE TO TEMPERATURE CHANGES, THE COMBINATION COMPRISING. ELECTROMAGNETIC WAVE SUPPORTING MEANS, MEANS FOR COUPLING ELECTROMAGNETIC WAVES INTO SAID WAVE SUPPORTING MEANS, A SPECIMEN OF A SINGLE CUBIC CRYSTAL OF GYROMAGNETIC MATERIAL THAT EXHIBITS GYROMAGNETIC EFFECTS TO SAID ELECTROMAGNETIC WAVES, SAID SPECIMEN BEING SUBJECT TO TEMPERATURE VARIATIONS, SAID SPECIMENT BEING POSITIONED WITHIN SAID WAVE SUPPORTING MEANS IN THE PATH OF THE MAGNETIC FIELD OF SAID WAVES AND BEING MAGNETICALLY POLARIZED IN A GIVEN DIRECTION, SAID SPECIMEN BEING CRYSTALLOGRAPHICALLY ORIENTED RELATIVE TO SAID GIVEN DIRECTION TO BE MAGNETICALLY POLARIZED PARALLEL TO ITS 110 PLANE AND AT AN ANGLE OF APPROXIMATELY 30 DEGREES TO ITS 100 AXIS THEREBY TO TEMPERATURE STABILIZE SAID ANISOTROPY FIELD, WHEREBY THE ELECTRICAL PROPERTIES OF SAID DEVICE ARE STABILIZED AGAINST SAID TEMPERATURE VARIATIONS. 